When such a battery is rechargeable, it is known that the battery must be prevented from discharging whenever its internal voltage drops below a certain threshold, otherwise the battery is damaged and the number of discharge/recharge cycles is reduced compared with the number initially possible. For that purpose, certain battery manufacturers have made provision to equip each of their batteries with a battery discharge control circuit that compares the output voltage of the battery with a threshold, and that disconnects the battery from the equipment whenever said threshold is crossed. The value of the threshold can be set permanently. In which case, the battery is properly protected, but battery use is not optimized. By way of example, when the electronic apparatus is a cellphone, the current output by the battery powering the cellphone is greater when the telephone is in call mode than when it is standby mode. In which case, the battery output voltage that is used in the comparison with the threshold is not in any way a correct reflection of the internal voltage of the battery, and the battery is disconnected from the cellphone too soon.
In order to protect the battery as it discharges while also optimizing use of the battery, it has been proposed to make provision for the discharge control circuit to include means for estimating the voltage drop due to the internal impedance of the battery on the basis of a measurement of the current output by the battery, and means for adapting the threshold below which the battery should be disconnected, the adaptation being a function of the measured output current of the battery. A control circuit operating on that principle is described in Document U.S. Pat. No. 5,705,913, for example.
The drawback with that above-described control circuit lies in the fact that it is installed inside the battery, which increases the cost of the battery. Naturally, that cost is passed on to the user each time the user has to buy a new battery. Furthermore, the addition of a control circuit in the battery is in contradiction with the constant search for improved battery compactness. Finally, if the user installs a rechargeable battery, and there is no discharge control circuitry in the portable apparatus, then it is not possible to protect the battery.